1. Field of the Invention
The present invention relates to a thin-film semiconductor substrate, a method of manufacturing a thin-film semiconductor substrate, a method of crystallization, an apparatus for crystallization, a thin-film semiconductor device, and a method of manufacturing a thin-film semiconductor device, which are applicable to, for instance, an active matrix flat panel display.
2. Description of the Related Art
Thin-film semiconductor technology is important in forming semiconductor devices, such as a thin-film transistor (TFT: Thin-Film Transistor), a contact image sensor and a photoelectric conversion device, on an insulative substrate. The thin-film transistor is a MOS (MIS) field-effect transistor and is applied to a flat panel display such as a liquid crystal display (see, e.g. non-patent document 1: “Flat Panel Display 96”, pp. 174–176).
In general terms, a liquid crystal display is characterized by thinness, lightness, low power consumption, and easy display in color. For these features, liquid crystal displays are widely applied to displays of personal computers or various portable information terminals. In the case where a liquid crystal display is of an active matrix type, thin-film transistors are provided as pixel switching elements.
An active layer (carrier drift layer) of the thin-film transistor is formed of, e.g. a silicon semiconductor thin film. Silicon semiconductor thin films fall into two categories: amorphous silicon (a-Si) and polycrystalline silicon (non-single-crystal crystalline silicon) with a crystal phase. The polycrystalline silicon is mainly polysilicon (poly-Si). Microcrystal silicon (μc-Si) is also known as polycrystalline silicon. Examples of semiconductor thin-film material, other than silicon, include SiGe, SiO, CdSe, Te, and CdS.
The carrier mobility of polycrystalline silicon is about 10 times to 100 times greater than that of amorphous silicon. This characteristic is very excellent as that of semiconductor thin-film material of switching devices. In recent years, attention has been paid to the thin-film transistor having an active layer formed of polysilicon, since it has high operation speed and is a switching device that can constitute various logic circuits such as a domino circuit and a CMOS transmission gate. This logic circuit is required in forming a drive circuit, a multiplexer, an EPROM, an EEPROM, a CCD and a RAM in a liquid crystal display device and an electro-luminescence display device.
A typical conventional process of fabricating a polycrystalline silicon semiconductor thin film will now be described. In this process, an insulative substrate of glass, etc. is first prepared. A silicon oxide film (SiO2), for instance, is formed as an undercoat layer (or buffer layer) on the insulative substrate. An amorphous silicon film (a-Si) with a thickness of about 50 mm is formed as a semiconductor thin film on the undercoat layer. Then, dehydrogenation is conducted to lower the hydrogen content in the amorphous silicon film. Subsequently, the amorphous silicon film is melted and recrystallized by, for instance, an excimer layer crystallization method. Specifically, an excimer laser beam is applied to the amorphous silicon film, thereby changing the amorphous silicon into polycrystalline silicon.
At present, the semiconductor thin film of the thus obtained polycrystalline silicon is used as an active layer of an n-channel or a p-channel thin-film transistor. In this case, the field-effect mobility of the thin-film transistor (the mobility of electrons or holes by field effect) is about 100 to 150 cm2/Vsec in the case of the n-channel thin-film transistor and is 100 cm2/Vsec in the case of the p-channel thin-film transistor. The use of such thin-film transistors makes it possible to obtain a driver-circuit-integrated display device wherein driver circuits such as a signal line drive circuit and a scanning line drive circuit are simultaneously formed along with pixel switching elements on the same substrate. The manufacturing cost of the display device can thus be reduced.
The electrical characteristics of present-day thin-film transistors are not so excellent that signal processing circuits, such as a D/A converter for converting digital video data to analog video signals and a gate array for processing digital video data, are integrated on the substrate of the display device. To achieve this, the thin-film transistor needs to have a current driving ability that is twice to five times higher than that of the present-day device. Further, the field-effect mobility needs to be about 300 cm2/Vsec or more. The electrical characteristics of the thin-film transistor need to be further enhanced in order to increase the functional performance of the display device and to enhance the added values. In the case where a static memory that comprises a thin-film transistor is added to each pixel, for example, in order to provide a memory function, the thin-film transistor needs to have electrical characteristics that are substantially equal to those of single-crystal semiconductor. Therefore, it is important that the characteristics of the semiconductor be enhanced.
As a means for enhancing the characteristics of the semiconductor thin film, it can be thought, for example, that the crystallinity of semiconductor thin film is made closer to that of a single crystal. In fact, if the entire semiconductor thin film on the insulative substrate can be made into a single crystal, it becomes possible to obtain characteristics that are substantially equal to those of a device using an SOI substrate, which is considered to be the next-generation LSI. This attempt was first made more than ten years ago as a 3D device research project, but technology for making the entire semiconductor thin film into a single crystal is yet to be established. Even at present, it is hopefully expected that the semiconductor grain in the semiconductor thin film is a single crystal.
In the prior art, techniques for growing a single-crystal semiconductor grain to a large size in the crystallization of an amorphous semiconductor thin film have been proposed (see, for instance, non-patent document 2: Journal of the Surface Science Society of Japan, Vol. 21, No. 5, pp. 278–287). Non-patent document 2 was published as an achievement of the research that had been energetically continued by Matsumura, et al. Non-patent document 2 discloses a phase-modulation excimer laser crystallization method wherein an excimer laser beam that is spatially intensity-modulated by a phase shifter is applied to an amorphous silicon thin film, thereby melting and recrystallizing the amorphous silicon thin film into a polysilicon thin film. In an ordinary laser crystallization method, an optical system that is referred to as “beam homonizer” is employed to uniformize the excimer laser intensity on the plane of the silicon thin film. On the other hand, according to the scheme of the phase-modulation excimer laser crystallization method, the excimer laser intensity on the plane of the silicon thin film is varied to have high and low levels by the phase shifter, and a temperature gradient corresponding to the resultant intensity distribution is provided in the silicon thin film. The temperature gradient promotes growth of a single-crystal silicon grain from a low-temperature region to a high-temperature region in a lateral direction that is parallel to the plane of the silicon thin film. As a result, compared to the prior-art laser crystallization method, a single-crystal silicon grain can be grown to a larger size. Specifically, the single-crystal silicon grain can be grown to a size of several microns, which can contain an active device such as a thin-film transistor. Therefore, it is reasonable to think that a thin-film transistor with electrical characteristics, which can meet the above-mentioned requirements, can be obtained by forming the thin-film transistor in this single-crystal silicon grain.
As stated above, the phase-modulation excimer laser crystallization method is effective in obtaining the large-size single-crystal silicon grain. However, as described in non-patent document 2, the large-size single-crystal silicon grain is surrounded by polysilicon or amorphous silicon, which is a countless number of small-size single-crystal silicon grains. If the thin-film transistor is formed with a displacement from the range of the large-size single-crystal silicon grain, the electrical characteristics of the thin-film transistor would considerably be degraded. If such a thin-film transistor is included in a product such as a flat panel display, the display would become a defective one.
In the prior-art, even if a glass substrate, which is covered with an amorphous silicon thin film, is placed at such a predetermined position as to face a phase shifter in a crystallization process for crystallizing the amorphous silicon thin film, the following problem would arise. That is, in a process subsequent to the crystallization process, a thin-film transistor, which is to be formed in the silicon thin film, cannot exactly be positioned within the range of the large-size single-crystal silicon grain.